Multiplying Preclinical Models for Multiple Myeloma
Authors: Melania Tesio
Journal: HemaSphere (2023)

Multiple myeloma (MM) is a clonal proliferation of bone marrow plasma cells. This disease develops in a step-wise manner from 2 well-defined premalignant states. The most precocious once, monoclonal gammopathy of undetermined significance (MGUS), is an asymptomatic condition defined by the presence of abnormal plasma cells clones. Following progressive acquisition of genomic abnormalities, MGUS evolves into smoldering multiple myeloma (SMM), a still asymptomatic intermediate stage presenting with more abnormal plasma cells, which eventually transforms into a clinically symptomatic MM.1 Accumulation of genetic lesions and evasion of immune surveillance are among the crucial factors that contribute to the disease development.2 Although the identification of genetic heterogeneity allowed a risk-based stratification of MM patients, the interplay between genetically different MM cells and immune surveillance mechanisms remains largely unknown. In clinical settings, distinct immunotherapy approaches have produced very distinct response rates, with checkpoint inhibitors (such as anti-PD-1 [programmed cell death protein 1] monoclonal antibodies) showing limited activity as single agents. Understanding how genetic heterogeneity shapes immune surveillance and immunotherapy responses is, therefore, crucial to tailor immunotherapies based on the specific pathogenetic features of the disease. To tackle this aspect, Larrayoz and colleagues3 undertook a herculean effort and developed 15 genetically engineered preclinical models, which enabled correlating MM genetic and immunological features with immunotherapy responses. To develop murine models mimicking key features of the human pathology (ie, genetic heterogeneity and the step-wise progression), the researchers first interbred 8 murine lines, each carrying a distinct MM driver lesion in mature germinal center B-cells. This strategy lead the authors to obtain 31 strains bearing 2 to 4 distinct genetic lesions. These lines were next screened for the development of plasma cells tumors reflecting MM main pathogenetic features. By doing so, the authors obtained a panel of 15 murine strains mimicking not only standard and high-risk genetic subgroups but also disease evolution from premalignancy. More in particular, within this panel, murine lines whereby MYC and IKK2-mediated NF-kB signaling were concomitantly activated (MI lines) developed an MGUS-like state, which rapidly transformed into an aggressive MM. In contrast, the murine lines arising from B-cell lymphoma 2 (BCL2) expression and IKK2-mediated NF-kB activation (BI lines) developed an MGUS-like state, which progressed to MM state with a slower kinetic and through an intermediate SMM-like stage (Figure).Figure.: The preclinical platform developed by Larrayoz and colleagues recapitulates MM step-wise development from premalignant states and allows interrogating immune evasion features affecting clinical responses to immunotherapies. MGUS = monoclonal gammopathy of undetermined significance; MM = multiple myeloma; SMM = smoldering multiple myeloma.In MI lines, the exogenous MYCtransgene drove a stable MYC expression over disease development, whereas in BI lines, the endogenous MYC expression increased as long as the disease progressed from MGUS to MM. As a consequence, at the MM stage, the transcriptomes of MI- and BI-derived strains were similarly enriched for MYC target genes. Interestingly, an increased MYC expression from the MGUS to the MM stage marked disease evolution in patients as well. In both murine and human MM cells, moreover, MYC was stabilized during disease evolution by the constitutive activation of the MAPK signaling, which was triggered by mutational events. The combined analysis of these novel murine models and patients’ samples hence demonstrated that the activation of a MAPK-MYC axis underlies disease progression from premalignancy regardless of genetic heterogeneity. In this scenario, MYC expression levels can distinguish cases with early and late progression from precursor stages. As further evidenced, an early MYC activation induced immune evasion by remodeling the bone marrow immune microenvironment. In MI lines rapidly developing MM, MYC pharmacological targeting reduced the expression of PD-L1 (programmed cell death ligand 1), a transmembrane protein, which upon binding to PD-1 expressed on T cells inhibit their responses. Because cytotoxic PD-1+CD8+ T cells were abundantly found in the bone marrow of MI mice at the MGUS stage, these data suggest that MYC activation in early stages is required to evade the response of these tumor-reactive cells by increasing PD-L1 expression on MM cells. Consistently, the delivery of an anti-PD-1 therapy at the MGUS stage significantly reduced tumor burden and delayed MM development in MI lines. In contrast, this therapy was ineffective in BI lines, which at the same stage of disease development showed a lower number of tumor-reactive cytotoxic PD-1+CD8+ T cells. In these mice, instead, a higher number of immunosuppressive CD4+PD-1+ regulatory T-cells (Treg) was detected. As such, a distinct ratio of bone marrow cytotoxic CD8+T cells/Treg cells was shown to characterize early versus late MM progression models (MI lines versus BI lines, respectively). Taken together, these data demonstrated that anti-PD-1 therapies are only effective in mouse models presenting an early MYC-driven progression and a higher CD8+ T-cells/Treg cells ratio, and they next promoted the authors to examine whether this ratio could predict disease evolution and therapies responses in clinical settings. Analysis of 69 untreated SMM patients showed that cases with a higher CD8+ T cells/Treg cells ratio exhibited a shorter progression to MM when compared with cases with a lower one, thus suggesting a potential benefit of PD-1 therapies in SMM patients at high risk of progression toward MM. A higher CD8+ T cells/Treg cells ratio moreover correlated with early relapse in newly diagnosed MM patients undergoing standard lenalidomide–dexamethasone therapies. Last but not least, the authors investigated whether altering the balance between CD8+ cytotoxic T cells and immunosuppressive Tregs could shape responses to immune checkpoint blockade. Interestingly, when increasing CD8+ T cells-mediated cytotoxicity or depleting Treg cells, the authors managed to reverse the resistance to anti-PD-1 and PD-L1 monoclonal antibodies and extend the animal survival. In summary, with this impressive study, Larrayoz and colleagues created a preclinical platform, which recapitulates and mimics key features of human MM pathogenesis, such as the step-wise development from premalignant states. By translating to patient settings, the mechanistic investigations conducted through this preclinical platform, the authors identified novel immune evasion features, which affect clinical responses to immune check point inhibitors. Being suitable to test immunotherapy responses, these preclinical models hold the promise to rapidly move the field forward. AUTHOR CONTRIBUTIONS MT conceived and wrote the article. DISCLOSURES The author has no conflicts of interest to disclose. SOURCES OF FUNDING The author has no sources of funding to disclose.